A solid oxide fuel cell (SOFC) is an electrochemical device that produces direct current electricity by electrochemical combination of a fuel with an oxidant. An SOFC uses an oxygen ion-conducting electrolyte to separate an air electrode (cathode) from a fuel electrode (anode). The fuel is oxidized at the anode and electrons are released to an external circuit where they are accepted by the cathode. The cathode reaction causes the oxidant gas to be reduced to oxygen ions, which then migrate across the oxygen ion-conducting electrolyte to the anode. The movement of electrons around the external circuit produces an electromotive force (typically 1 volt for a single cell). By the application of a load across the cell, current flows, thus producing a power density, the value of which depends upon the design of the cell and the materials used. A cell typically runs at between 700 and 1000 degrees Celsius. Science and Technology of Ceramic Fuel Cells, by N. Q. Minh and T. Takahashi, Elsevier, Amsterdam, 1995 (incorporated herein by reference), further describes the principle reactions in an SOFC, and the methods by which electricity can be produced.
FIG. 1 shows a typical arrangement of multiple tubular fuels cells 2. The main body of each cell 2 is typically a composite metallic ceramic that is extruded into a tube shape and then sintered. In some applications, the tube may be a nickel-coated ceramic. This ceramic tube acts as an anode. An electrolyte layer, such as yttria-stabilized zirconia (YSZ), is applied over the outer surface of the anode, and a cathode layer, such as a doped lanthanum manganate (LaMnO3) is applied over the electrolyte. The outer cathode is surrounded by an oxygen-containing gas such as air. In a typical system, each fuel cell might be 1-50 mm in diameter and 200-2000 mm long.
The fuel cells 2 are attached to a support manifold 5 by end caps 14. Typically, the outer cathode layer does not extend the entire length of the fuel cell 2, but rather stops somewhat short of the end cap 14 so that the end cap is mechanically coupled to the anode or electrolyte layer. Physical support of the fuel cells 2 may rely on support of the electrode structure (i.e., anode) or on support provided by the electrolyte. The support manifold 5 contains a fuel distribution plenum that delivers a fuel mixture into the interior of each fuel cell 2. The hot exhaust from the fuel cells 2 exits out the other end through a perforated plate 24 (held by support rod 26) into an exhaust plenum. Typically, the exhaust plenum also contains a heat exchanger that uses the heat of the exhaust to pre-heat the incoming air that is delivered to the exterior of the fuel cells 2.